As fans of the pair know, Klum and Kaulitz's love story dates back to early 2018.

The supermodel-mom to daughters Lou, 10, and Leni, 15, and sons Johan, 12, and Henry, 14-and the Tokio Hotel musician, 29, first sparked dating rumors in March 2018 after they were spotted kissing on set while she was filming America's Got Talent.

"Lately, I'm being reminded more about my age by people other than myself," Klum toldInStyle in July 2018. "My boyfriend is many years younger than me, and lots of people are questioning that and asking about it. That's really the only time when age seems to be shoved in my face and I have to give an answer for it."

"I don't really think about it that much otherwise. You have to just live a happy life without worrying too much about what people think because worrying is only going to give you more wrinkles," Klum added to the outlet.

In September 2018, Klum opened up her relationship with the rocker again at the America's Got Talent semifinals live show screening and red carpet, calling Kaulitz "absolutely wonderful."

https://www.instagram.com/p/BzWo10CpPyA

"He is the kindest, sweetest, most caring, loving person. I feel so lucky. I don't know the last time I was this happy in my life," she told PEOPLE. "He is absolutely wonderful. Maybe it is because he is German and we understand each other in a different way. But so far so good."

"I don't know why everyone keeps saying that. Everyone is asking me. When you do what I do, you wear a lot of fashion. A lot of jewelry. I always look for something that goes with the outfit. So, I wear big rings," the America's Got Talent judge told PEOPLE in September.

Things didn't stay that way for long, however, as the couple were engaged just three months later. Klum announced the happy news on Instagram and Twitter in December. "I SAID YES," she captioned a black-and-white portrait of herself and Kaulitz as she showed off her new engagement ring.

At the time, a source close to the duo told PEOPLE that Klum is "so in love" with Kaulitz.

"They went to Mexico before their engagement, around Thanksgiving, and that's when he started talking to the kids about his plans in secret and it was so hard for them to not tell anyone. They were so excited and happy for their mom," a friend of Klum shared with PEOPLE.

One month later, on Christmas Eve morning, "Tom went to the kids and asked for Heidi's hand in marriage and they said 'yes,'" the friend shared.

"So Tom and the kids together worked on making a breakfast tray for Heidi that had coffee, flowers, and a ring box," said the friend. "It was important for Tom to have the kids included in the whole thing, so they all brought the tray up to Heidi's room and surprised her with breakfast in bed and then he proposed."

The breakfast-in-bed proposal couldn't have been any more perfect for the supermodel.

"She loved his thoughtfulness and that he included the kids," the friend explained. "They all surprised her together and she loved how he made the kids feel a part of it."

And without hesitation, the former Project Runway host, of course said "yes" to spending forever with Kaulitz.

"She was totally surprised and said yes right away!" said Klum's friend.

This is Klum's third marriage. Prior to Kaulitz, the supermodel was married to celebrity hairstylist Ric Pipino from 1997 to 2002 and singer Seal from 2005 to 2014. Kaulitz was also previously married once to Ria Sommerfeld from 2015 to 2018.